Making Test Bricks

To figure out a good mix for your soil, make up some test bricks. Try various
recipes and combinations, making sure you label each one carefully so you can
reproduce the best ones.

Make your first couple of test bricks out of the soil from the site or from
where you've dug out for the foundation. If you're lucky, you may have a soil
that works for cob as it is without adjusting the sand/clay ratio. After the
jar test, you'll have some idea what you might want to add for the other test
bricks. Make tests adding more and more sand until you've stretched it to the
point of too much sand. You'll be able to recognize that point because the
mix won't hold together well and will be hard to form into bricks. The sand
will slough off when you rub the dried brick. You can take the experiment to
the opposite extreme and add too much clay so the brick cracks while drying.
You will learn a lot doing these tests. Make three samples of each sand/clay
mix, adding different amounts of straw until you get a feel for how much you
need.

One easy way to check the clay shrinkage in your test bricks is to bury in
a stick the same length as the brick. The cob might shrink but the stick won't.
You can write the recipe for that brick on the stick with a permanent felt pen.

When the bricks have dried completely, have a look at them. Eliminate
the ones with the biggest cracks because they have too much clay in them. Does
the sand come off when you rub the bricks? If so, those have too much sand.
Try breaking the others. You can drop them from one or two feet up to test their
strength. The ones that are hardest to break are the best recipes. Leave them
out in the weather or simulate some rain and observe which ones are the most
resilient. Analyze what amounts of straw made the strongest bricks. Now you
have a good basic recipe. Lots of variations will work just fine, so don't
be too much of a perfectionist about all this. You may want to stretch the
recipe so you can use as big a proportion as possible of the soil you have on-site.

If you want to reassure yourself and do a more extensive test, build a garden
wall, an outdoor sculpture, or a bench, and observe how hard it dries and how
well it weathers. Remember, your walls will be protected from the rain by a
roof.